The oxidation and degradation of metals used in aerospace, commercial, and private industries are a serious and costly problem. To prevent the oxidation and degradation of the metals used in these applications, an inorganic protective coating can be applied to the metal surface. This inorganic protective coating, also referred to as a conversion coating, may be the only coating applied to the metal, or the coating can be an intermediate coating to which subsequent coatings are applied.
Chromate based coatings are currently used as inorganic conversion coatings because they provide corrosion resistant properties and adhesion for application of subsequent coatings. However, due to environmental concerns over chromium based compounds in the environment, there is a need for an environmentally safer replacement for chromate based conversion coatings. There is also a need for environmentally safer conversion coatings that can provide corrosion resistance to an underlying metal surface and adhesion to subsequently applied coatings.
Cerium and other rare earth element containing coatings have been identified as potential replacements for chromate based coatings in metal finishing. These coatings include cerium and other rare earth element containing coatings that are formed by various processes such as immersion, electroplating from a cerium nitrate solution, plating from an acidic cerium chloride containing solution and an oxidant (at elevated temperatures), as well as multi-step processes, and electrolytic and non-electrolytic processes having a sealing step. Further information on such coatings can be found in: Hinton, B. R. W., et al., Materials Forum, Vol. 9, No. 3, pp. 162-173, 1986; Hinton, B. R. W., et al., ATB Metallurgie, Vol XXXVII, No. 2, 1997; U.S. Pat. Nos. 5,582,654; 5,932,083; 6,022,425; 6,206,982; 6,068,711; 6,406,562; and 6,503,565; U.S. Patent Application Publication No. US 2004/0028820 A1; and PCT Application Publication No. WO 88/06639.
However, at least some of the coatings prepared using these compositions and methods do not perform as well as those formed using chromate treatments and/or can develop blisters on the surface and exhibit poor adhesion. Further, at least some of the prior art cerium and other rare earth element containing coatings can also suffer from one or more of the following disadvantages: (1) a tendency of the rare earth element to precipitate in solution away from the metal surface in the form of a sludge-like material; (2) difficulty in obtaining a uniform coating which does not tend to over-coat and exhibit poor adhesion to the substrate; (3) the necessity to use multiple steps and extensive periods of time to deposit a coating; (4) the use of commercially unattractive steps, such as sealing and/or the use of elevated temperature solutions; and (5) the necessity to use specific pretreatments and solution compositions in order to coat multiply alloys, especially aluminum 2024 alloys.
The ability to deposit a cerium-based conversion coating composition on the surface of a high copper-containing aluminum alloy, such as aluminum 2024, which is thick enough to provide corrosion protection can be problematic. Known coating compositions often exhibit poor adhesion or require the use of multiple steps and/or elevated temperature solutions to deposit the coating composition on the alloy. Specific deoxidizers have been used to more uniformly coat the metal substrate. However, when industrially accepted cleaners and deoxidizers are used on the alloy, the surface of relatively high copper-containing aluminum alloys has a tendency to pit and corrode as the cerium-based coating composition is deposited on the alloy. The rate of the undesired pitting can be more extensive than the ability of the cerium to deposit onto the alloy, resulting in visual pits across the alloy surface.
Therefore, there is a need for a conversion coating that can replace chromate based conversion coatings and that overcomes several of the deficiencies, disadvantages and undesired parameters of known replacements for chromate based conversion coatings.